Electronic scales for weighing articles such as letters and parcels are well known. Typically, in such a scale, a strain gauge transducer is arranged to respond to the weight of an article on a platen so as to provide a signal indicative of the weight of the article to a display device such as a numeric readout. However, strain gauges are expensive and difficult to mount (e.g., glue) to a structure with accuracy.
U.S. Pat. No. 4,629,019, entitled WEIGHT SCALE UTILIZING A CAPACITIVE LOAD CELL, discloses in FIGS. 8 and 8A a so called differential capacitor load cell having two coaxial, spaced-apart conductive cylindrical electrodes 128, 128' and a coaxial, conductive piston 124 disposed within the cylindrical electrodes in response to a load on a spring loaded platform 110. A capacitance is formed by the piston and each of the cylindrical electrodes. The piston is grounded, and the cylindrical electrodes are alternately connected (multiplexed) to the input of an oscillator circuit, FIG. 6, the output of which is a signal whose frequency is dependent on the position of the piston relative to the rings. The multiplexing allows for temperature compensation, but the arrangement is prone to fringing (edge) effects.
U.S. Pat. No. 3,702,957, entitled VARIABLE CAPACITANCE DISPLACEMENT TRANSDUCERS, discloses a capacitive position transducer comprising first and second capacitor plate electrodes disposed adjacent to each other, a common capacitor plate electrode disposed uniformly spaced from said first and second plate electrodes, and a screen guided for movement in the space between said first plate electrode and said common plate electrode whereby the ratio of capacitances between said first plate electrode and the common plate electrode and said second plate electrode and the common plate electrode varies in accordance with the position of said screen.
U.S. Pat. No. 4,561,038, entitled TRANSDUCERS, discloses a transducer comprising a capacitor incorporating two sets of rigid, electrically conductive plates interleaved one with the other, with spacing between them, the two sets of plates being movable relative to each other in a direction which maintains the spacing substantially constant, and the shape and/or disposition of the plates being such that when an oscillating voltage is applied across the capacitor an output signal derived from an intermediate plate varies with changes in the relative positions of the two sets of plates. The plates are preferably planar with relative movement being arranged to take place in the planes of the plates.
U.S. Pat. No. 3,566,222, entitled CAPACITIVE DISPLACEMENT TRANSDUCER, discloses two cylindrical coaxial reference electrodes (1 and 2) disposed side by side, and a coaxial cylindrical reference electrode (3) disposed within the reference electrodes for movement along the axis to vary the capacitive couplings between the moving electrode and the respective reference electrodes as a function of position. An oscillator (30) is connected to the reference electrodes and across an inductive potential divider (31). The divider or the movable electrode is adjusted to establish an "earth potential" position for the movable electrode (see column 3, lines 51-65).
In the aforementioned, '019, '957, '038 and '222 Patents, two or more capacitances are formed by three or more conductive elements. However, the "spacing", per se, of the conductive elements remains fixed.
U.S. Pat. No. 4,461,363, entitled HIGH-SPEED CAPACITIVE WEIGHING METHOD AND APPARATUS, discloses a set of fixed plates (12 and 16), forming a capacitor, through which an object is dropped to determine its mass. A separate, reference capacitor is involved.
U.S. Pat. No. 4,679,643, entitled CAPACITIVE WEIGHING DEVICE, discloses a capacitive plate electrode supported on a load-receiving spring plate and a fixed capacitive plate electrode. The spacing between the two capacitive plate electrodes varies in response to a load.
U.S. Pat. No. 4,656,871, entitled CAPACITOR SENSOR AND METHOD, discloses a circuit responsive to the capacitance changes in a variable, parameter responsive capacitor, C.sub.s.
U.S. Pat. No. 4,524,840, entitled ELECTRONIC SCALE WITH NON-PARALLEL CAPACITANCE MEANS, discloses a single capacitor transducer in a scale wherein two plates a fixed plate and a movable plate are disposed so as to be nonparallel to each other over the entire range of movement of the movable plate.
U.S. Pat. No. 4,308,929, entitled INTEGRAL PARALLELOGRAM CHARGE RECEIVER AND CAPACITIVE TRANSDUCER discloses a capacitive transducer formed by two plates 320 and 343 within a deformable parallelogram structure. The spacing of the plates varies in response to a load Q applied to a side of the parallelogram structure, but the plates 320,343 do not remain parallel when their spacing changes (see FIG. 1A).
U.S. Pat. No. 2,547,780, entitled CAPACITIVE PICKUP FOR PRESSURE INDICATORS, discloses a bridge circuit for determining the output of a capacitive pressure sensor.
The aforementioned '363, '643, '871, '840, '929 and '780 patents disclose single capacitor transducers.
U.S. Pat. No. 4,523,474, entitled CAPACITIVE PRESSURE SENSOR discloses a parallel plate arrangement for sensing pressure. The spacing of the plates changes in response to applied pressure. Multiple sets of parallel plates may be employed.
U.S. Pat. No. 4,661,768, entitled CAPACITANCE TRANSDUCING METHOD AND APPARATUS, discloses a circuit for determining the capacitance of an unknown capacitor.
U.S. Pat. No. 4,683,754, entitled CAPACITIVE DISPLACEMENT TRANSDUCER, discloses a pair of differential capacitors formed with a movable electrode disposed between two outer, fixed electrodes. The oscillation frequency of a self-excited oscillator circuit is counted by a counter and the output level of the counter is reversed each time a certain count is attained to cause the differential capacitances to be switched alternatively, thereby eliminating the effect of fixed (stray) capacitances formed across the differential capacitors. The movable electrode is shown in FIG. 1 to be acted upon by a linkage 19 passing through holes in the fixed outer plates to pressure sensitive diaphragms 11 and 12. Having holes through the outer plates i.e. "rings" tends to cause more fringe effects, and the switching arrangement of the '754 patent is a somewhat complex approach to eliminating the effect of stray capacitances.
Offenlegungsschrift No. 1,909,979, entitled KAPAZITIVER DRUCKGEBER (Capacitive Pressure-Sensor), discloses a metal plate 4 interposed in parallel between two metal rings 6 and 7. The spacing of the plate 4 changes differentially with respect to the plates 6 and 7. A hole through the metal rings 6 and 7 is required in this embodiment--much like it was in the '754 Patent.
U.S. Pat. No. 4,572,006, entitled LOAD CELLS, discloses a capacitive transducer arrangement wherein, e.g. in FIGS. 1 and 3, two capacitors are formed within a parallelogram flexure. One capacitor is a reference capacitor having a relatively constant valve, while the other capacitor varies in capacitance according to a load, P, along a side of the flexure. In FIG. 4, a differential capacitor arrangement is shown. In this case a movable element 11 has a pair of capacitor electrodes 23 and 24 attached to its upper and lower faces which are adjacent to a pair of fixed capacitor electrodes 26 and 27. The electrodes 23 and 24 are not indicated as being connected to each other and occupy only a small portion of the available space within the flexure. The area of the electrodes is not much larger than their spacing.
An Article entitled CAPACITIVE TRANSDUCERS (p77 et seq of November 1964 Instruments & Control Systems) discloses at page 80 the underpinnings of the above described differential capacitor arrangements, such as are disclosed in the aforementioned '754, '979 and '006 patents.
U.S. Pat. No. 4,237,989, entitled CAPACITIVE LOAD CELL AND METHOD OF MANUFACTURE, discloses a dual capacitor arrangement wherein two plates 26 and 27 are mounted to either face of a central, movable member of a flexure, and are spaced apart from two outer plates, 33 and 34 quite like the FIG. 4 embodiment of the '006 patent. As shown in FIG. 9 of the patent, two variable capacitors 31 and 32 are formed thereby, each capacitor affecting the frequency of an oscillator 91 and 92, respectively.